HAPS are fragile due to weight constaints, e.g. NASA’s Helios. Biplanes are more robust.

Although biplanes have high drag, HAPS only need to move slowly because at their operating height the wind speed is less than 50 mph, and to maintain station they only need to overcome that.

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    $\begingroup$ Possible duplicate of Can biplane or triplane designs be revived with modern materials? $\endgroup$ – fooot Jul 18 '19 at 17:52
  • $\begingroup$ I don't think this is a dupe. It has nothing to do with the construction materials available today vs in 1919 (with the exception of solar panels). It appears to be asking why the biplane format is not used for this category of aircraft. $\endgroup$ – FreeMan Jul 18 '19 at 19:19
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    $\begingroup$ That said, there would be a considerable loss of sun-facing wing surface by shortening the wing and placing half of it (roughly) in the shade below the other half, so that may be sufficient reason to not use a biplane design. $\endgroup$ – FreeMan Jul 18 '19 at 19:20
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    $\begingroup$ HAPS are fragile due to weight constaints. Biplanes are more robust > Robustness is heavy and biplanes are aerodynamically inefficient. $\endgroup$ – AEhere supports Monica Jul 19 '19 at 11:06

A biplane is more robust than a monoplane per unit weight. This is because the monoplane supporting beam has to live inside the max vertical depth of the wing. With a biplane the beam can be as deep as the vertical distance between the upper and lower wing. The biplane has a worse lift to drag ratio than equiv monoplane. But drag increases dramatically with air speed. As the HAPS is only slow flying maybe the drag is acceptable. At high altitude the Reynolds number decreases, this raises stall speed. Stall speed goes down with lower wing loading (plane weight / wing area). Biplane wing loading is lower so will have a lower stall speed. Of course it's all trade offs. These are arguments for the biplane.

  • $\begingroup$ I've just realised recreational gliders have most of the same constraints, and there are no biplane gliders. $\endgroup$ – Desmond Campbell Jul 25 '19 at 16:08

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